3GPP Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNodeB's or eNBs by 3GPP) to mobile stations (referred to as user equipment, or UEs, by 3GPP) are sent using orthogonal frequency-division multiplexing (OFDM). The signal transmitted by the eNB in a downlink (the radio link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. Accordingly, to demodulate any transmissions on the downlink, a UE relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their position in the time-frequency resource grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
Multi-antenna techniques used in LTE include the use of “transmit precoding” to direct the transmitted energy towards one particular receiving UE. With this technique, several antenna elements are used to transmit the same message, but individual phase and possibly amplitude weights are applied at each transmit antenna element. This is sometimes denoted UE-specific precoding and the RS in this case are denoted UE-specific RS. If the transmitted data is precoded with the same UE-specific precoding as the UE-specific RS, then the transmission is performed using a single virtual antenna, i.e., a single antenna port, and the UE need only to perform channel estimation using this single UE-specific RS and use it as a reference for demodulating the data in the corresponding resource block (RB).
UE-specific RS in a given RB pair are transmitted only when data is transmitted to a UE in the RB pair; otherwise they are not present. In LTE, UE-specific RS are included as part of each RB that is allocated to a UE for demodulation of physical downlink shared data channel (PDSCH). Up to 8-layer PDSCH transmission is supported, and therefore there are 8 orthogonal UE-specific RS, as described in 3GPP TS 36.211 (available at www.3gpp.org). These 8 different UE-specific RS correspond to antenna ports 7-15 in the 3GPP specifications.
Release 11 of the 3GPP standards for LTE include specifications directed to so-called coordinated multi-point transmission (CoMP). To support CoMP, it has been decided that a UE can be semi-statically configured with a reference signal sequence for the UE-specific RS (antenna ports 7-15) in a UE-specific manner, where the initialization values for the scrambling generator are available for dynamic selection. In this case the dynamic selection of the reference signal sequence is signaled in the downlink control information transmitted in the downlink control channel. This is useful in a shared cell scenario, where the same cell ID, NIDcell, is used by a group of geographically distributed nodes, which group often includes a macro node and all picos that are essentially within the coverage area of the macro. Depending on the channel properties, there is more or less interference between the UE-specific RS used in the two pico nodes. Therefore, it is useful to configure the reference signal in a UE-specific manner instead of a cell-specific manner.
In the Release 10 specifications for LTE, a relay control channel was defined. This relay control channel, denoted R-PDCCH, is for transmitting control information from an eNB to one or more relay nodes. The R-PDCCH is placed in the data region and is thus similar to a PDSCH transmission. The transmission of the R-PDCCH can be configured to use either a common reference signal (CRS) to provide wide cell coverage or relay node (RN)-specific reference signals to improve the link performance towards a particular RN by precoding, in a manner that is similar to how the PDSCH is transmitted with UE-specific RS. In the latter case, the UE-specific RS is used for the R-PDCCH transmission. The R-PDCCH occupies a specific number of configured RB pairs in the system bandwidth and is thus frequency multiplexed with the PDSCH transmissions filling in the remaining RB pairs.
In LTE Release 11 discussions, attention has turned towards adopting these same techniques to support enhanced control channels, including enhanced versions of PDCCH, PHICH, PCFICH, PBCH. Thus, the same principle of UE-specific transmission as discussed above for the PDSCH and the R-PDCCH is applied to the enhanced control channel, thus allowing the transmission of generic control messages to a UE based on UE-specific reference signals. These enhanced control channels are commonly known as the enhanced PDCCH (ePDCCH), enhanced PHICH (ePHICH), and so on.
More particularly, it has been agreed to use antenna ports p∈{7, 8, 9, 10} for demodulation of the enhanced control channels. These are the same antenna ports that are used for PDSCH transmissions based on UE-specific RS. This enhancement means that precoding gains can be achieved also for the control channels. Another benefit is that different RB pairs (or enhanced control regions) can be allocated to different cells or different transmission points within a cell, and thereby inter-cell or inter-point interference coordination between control channels can be achieved.
Alternatively, the same enhanced control region can be used in different transmission points within a cell or in transmissions from transmission points in different cells, but which are not highly interfering with each other. A typical case is the shared cell scenario, where a macro cell contains lower power pico nodes within its coverage area, the pico nodes having (or being associated to) the same synchronization signal/cell ID as the macro node. In pico nodes that are geographically separated, the same enhanced control region, i.e., the same physical resource blocks (PRBs) used for the ePDCCH, can be re-used. In this manner the total control channel capacity in the shared cell will increase, since a given PRB resource is re-used, potentially multiple times, in different parts of the cell.
The specifications for enhanced control channels developed by 3GPP contemplate a wide variety of scenarios in which the enhanced control channels will be used. As a result, improved techniques are needed for assigning reference signal sequences to achieve robust channel estimation in these various enhanced control channel scenarios.